I. Field of the Invention
The present invention relates to communication. More particularly, the present invention relates to a novel and improved method detection and avoidance of a serial vocoder connection.
II. Description of the Related Art
In digital communications systems, a transmitter may employ a vocoding system which efficiently encodes voice information. One method of efficient encoding uses variable data rate encoding to lower the amount of data sent during pauses or other absences of voice activity, thereby reducing the level of interference caused by this transmitter to receivers other than the intended receiver. At the receiver, a vocoding system is employed for reconstructing the voice information. In addition to voice information, non-voice information alone or a mixture of voice and nonvoice information may be transmitted to the receiver.
A vocoder which is suited for application in this environment is described in copending U.S. patent application Ser. No. 07/713,661, now issued U.S. Pat. No. 5,414,796, issued May 9, 1995 entitled "VARIABLE RATE VOCODER," filed Jun. 11, 1991 and assigned to the assignee of the present invention. This vocoder converts digital samples of the voice information to encoded data at four different rates, e.g. approximately 8,000 bits per second (bps), 4,000 bps, 2,000 bps and 1,000 bps, based on voice activity during a 20 millisecond (ms) frame. Each frame of vocoder data is formatted with overhead bits as 9,600 bps, 4,800 bps, 2,400 bps, and 1,200 bps data frames. The highest rate data frame which corresponds to a 9,600 bps frame is referred to as a "full rate" frame; a 4,800 bps data frame is referred to as a "half rate" frame; a 2,400 bps data frame is referred to as a "quarter rate" frame; and a 1,200 bps data frame is referred to as an "eighth rate" frame.
Additional details on the formatting of the vocoder data into data frames are described in copending U.S. patent application Ser. No. 07/822,164, now issued U.S. Pat. No. 5,504,773 issued Apr. 2, 1996, U.S. Pat. No. 5,511,073 issued Apr. 23, 1996 and U.S. Pat. No. 5,568,483, issued Oct. 22, 1996 entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION," filed Jan. 16, 1992 and assigned to the assignee of the present invention. The data frames may be further processed, spread spectrum modulated and transmitted as described in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Apr. 7, 1992, and assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
A system employing an efficient vocoder of the type described by the above mentioned patents and patent applications provides high quality when coupled with a system which does not employ efficient vocoding such as a standard land line system. However when a vocoded system is coupled to another vocoded system such that a tandem (back-to-back) vocoder arrangement results, the voice quality is degraded over the single vocoder system case. This present invention seeks to avoid the disadvantage of tandem vocoding by detection and elimination of the arrangement.